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Common drugs

Various drugs are available to treat viral infections. Although these drugs may not eliminate an infection, they are often effective in reducing its severity. Many viral illnesses are mild and clear up without treatment because healthy people can usually fight off infection quickly. Sometimes, antiviral drugs help to relieve symptoms and hasten recovery. However, because viruses invade body cells in order to multiply, antiviral drugs can damage body cells as well as the targeted viruses. Their use is therefore usually limited to treating severe or recurrent infections. Some antiviral drugs are more effective against certain viruses than others. A particular range of antivirals is used to treat HIV infection (see Drugs for HIV infection and AIDS).

Why are they used?

The main use for antivirals is in the treatment of infections that recur or persist over a prolonged period, such as herpes, HIV, and chronic hepatitis B and C. Some drugs are also effective in treating or preventing infections in people with impaired immune systems who are at increased risk of severe illness and complications.

Antivirals such as aciclovir, valaciclovir, and famciclovir are most commonly used to treat herpes simplex infections in people who have severe or recurrent genital herpes or cold sores. These drugs are also effective against the varicella–zoster virus, which causes chickenpox as well as shingles (see Herpes zoster).

Some antivirals are used for specific infections, especially in at-risk people such as those whose immunity is impaired by disease or by drug treatment (such as chemotherapy) and those with certain conditions such as diabetes, chronic kidney disease, and significant cardiovascular disease. For example, oseltamivir and zanamivir may be used to treat or prevent influenza in at-risk people, and to treat otherwise healthy people in certain situations, such as pandemics. Ganciclovir is used for preventing or treating cytomegalovirus infection, especially in people with reduced immunity. Ribavirin may be used to treat babies with a severe form of bronchiolitis. It may also be used to treat Lassa fever (see Viral haemorrhagic fevers) and in combination with interferon alfa to treat chronic hepatitis C infection. Lamivudine and interferon alfa are used in the treatment of chronic hepatitis B infection.

How do they work?

Viruses invade body cells, where they use human genetic material (DNA) to reproduce. Antiviral drugs act in various ways to block this process, either by causing changes within body cells to prevent the virus from replicating or by preventing the virus from entering cells. If treatment is started early, the drugs usually work rapidly and relieve symptoms within a few days. However, some viruses develop resistance to the effects of drugs, which can make particular infections difficult to treat.

How are they used?

The drugs used to treat herpes infections, such as aciclovir, can be applied as a topical preparation or taken orally, depending on the site and severity of the infection, or given by injection for urgent treatment of a serious infection. For mild infections, topical antivirals can be bought over the counter.

Ganciclovir and interferon are given by injection in hospital. Ribavirin can be given by inhaler to babies with bronchiolitis, by injection for Lassa fever, and orally for hepatitis C. For influenza, zanamivir is given by inhaler, and oseltamivir is given orally.

What are the side effects?

Aciclovir, valaciclovir, and famciclovir do not usually produce side effects, although when taken orally they may cause nausea. Rarely, when aciclovir is given by injection, the drug can cause kidney damage and may produce symptoms such as confusion and seizures. Ganciclovir may cause skin rashes and nausea. This drug can reduce red blood cell production, causing anaemia, and white blood cell production, increasing susceptibility to further infections. Ganciclovir may also lead to impairment of kidney function. The side effects of oseltamivir include nausea, vomiting, and other intestinal disturbances, and sometimes headache, insomnia, and dizziness. Zanamivir rarely produces side effects, although it may cause breathlessness and wheezing in some people, so it is used with caution in those with existing respiratory problems such as asthma. Ribavirin can cause anaemia and fetal abnormalities.

Interferon alfa commonly produces side effects, including nausea and flu-like symptoms such as tiredness, fever, and aching muscles. These effects are usually mild and may diminish with continued treatment. However, prolonged treatment can lead to decreased production of red and white blood cells and also depression.